My first post here, figured I could post a little insight.
I just setup a Freightliner FL112 with a Cummins and 22ft box. Tandem rear, so I can haul 54000 lbs of stuff around. Now many might think I am crazy, but here is why I went this way
1. Water is heavy, I have to carry every drop with me to site, some sites can be an hour out, some can be closer. To drive all that way with 4 hours of water available is costly for the refill and can take a lot of time. Also you fill up once for the day and depending on what nozzle you use you can be going for a long while. I can also run 2 people I figure for a total of 7 hours with a 5.0 nozzles on each gun.
2. Alot of work requires me to go offroad into the mud as I do a lot of heavy equipment. The full lockers on the back will save my truck from getting stuck and having to get pulled out by the equipment I am cleaning. Don't laugh it has happened as as much as I avoid it by walking the worst routes I have missed things before and been stuck.
3. Weight, since I have to go through scales I don't like the idea of wondering if I am going to be safe or over. I am now have a 10,000 lb buffer, so I can hit any buffet before work and not be worried!
4. Wear and tear, every guy I talked to told me it is better to have an over engineered truck as you will break things more often when they are put to the limit.
5. I can put everything I use or need in the truck pretty much. Surface cleaners, chemicals, all the different wands, and eventually when needed the reclaim system. I can start my day doing one thing, and get a call half way through the day and be doing something else
6. Refilling can be a big pain. There are only 4 places around the city to refill the bulk water tank and many times there can be a wait of up to 2 hours. Been half way through a job and had to take a 2 hour break waiting for water does not look good on you and typically the customer is not very happy. 1 fill and she is all complete for the day most the time.
In terms of the truck mentioned, I was told to stay away from Cat engines in trucks. My construction guys that had all cat equipment on their site said they would never touch the cat engines in the trucks. Only exception was an old 3406 with no electrics I was told. They are throw away engines I was told, and especially the 3126's with the fuel system they have.
In terms of size, if you have to haul water bigger is better. Also size the truck to what your market dictates. If you do residential and or jobs that don't have you being at one place for a long time then a smaller truck is probably better. If you have many jobs that are large or multiple people jobs bigger is better.
Good luck with the FL70!